Switched light element array and method of operation

ABSTRACT

A switched light element array includes first, second and third light emitting elements, and first and second switches. The first light emitting element includes first and second terminals, and the second light emitting element includes a first terminal, and a second terminal coupled to the second terminal of the first light emitting element. The third light emitting element includes a first terminal coupled to the first terminal of the first light emitting element and a second terminal. The first switch includes a first terminal coupled to each of the first terminals of the first and third light emitting elements and a second terminal coupled to the first terminal of the second light emitting element. The second switch includes a first terminal coupled to the second terminal of the third light emitting element, and a second terminal coupled to each of the second terminals of the first and second light emitting elements.

This application is a national stage application under 35 U.S.C. §371 ofInternational Application No. P PCT/IB07/53821 filed on Sep. 20, 2007,which claims priority to European Application No. 06121876.4, filed onOct. 6, 2006, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to devices employing lightemitting elements, and more particularly to light element arrays,devices, and methods of operating same.

BACKGROUND OF THE INVENTION

Light emitting elements, such as light emitting diodes (LEDs), enjoyincreasing use in a wide range of applications, some examples being backlight sources in liquid crystal displays, flashes for charge coupleddevice cameras, general lighting, as well as other applications. In manyof these applications, LEDs of different colors are used, e.g. arrangedin an array, to produce various color points. The operating conditionsfor the LED array may be equally as diverse as the array's application,such operating conditions requiring, e.g., low power, high operatingtemperature, and fast LED activation and deactivation times.

Typically, each LED array is powered by a driver circuit operable in oneof several different driving modes depending upon the desired lighteffect. The LED driver circuit may be driven in a constant current mode,whereby the LED array is supplied a constant current to provide light ata constant intensity. The LED driver may also operate in a variablecurrent mode, whereby the LED array is supplied a varying current toproduce a varying intensity of light. The LED driver may also operate ina pulse width modulation (PWM) mode, whereby the LED array is suppliedusing a PWM current waveform in which the on period of the PWM waveformdetermines the time period over which the LED array is activated, andthus determines the light output and thus the color point of the LEDarray. The PWM mode may be implemented with either the constant currentmode or the varying current mode to provide a combination of each ofthese attributes, i.e., constant or varying light intensity.

Unfortunately, a large number of circuit components are needed toprovide the aforementioned functionality. For example, when a constantcurrent, PWM mode of operation is desired, at least one current sourcefor the LED array and one switch for each LED in the array is typicallyrequired. In the case in which a varying current mode of operation isdesired, a complex current source operable to quickly change currentlevels is required. In the case in which a varying current, PWM mode ofoperation is desired, a complex current source and one switch per LEDwithin the array is usually required.

A high part count for operation and control of the LED array degradesLED performance in a number of ways, each component increasing powerconsumption of the LED array and contributing parasitic effects whichoperate to reduce activation and deactivation times of the LEDs.Furthermore, when the LED array is implemented in a high temperatureapplication, each component will require a high temperature rating, acapability that further increases the cost for each required component.Acknowledgement of the problems associated with high part count LEDdrivers can be seen in U.S. Pat. No. 5,736,881 to Ortiz disclosing anPWM LED driver and LED array configuration in which one current sourceis used to control multiple LED strings.

SUMMARY OF THE INVENTION

Accordingly, it may be desirable to provide a light element array andmethod of operation which can provide control of separate light emittingelements within an LED array, and which requires fewer circuitcomponents.

This and other aspects of the invention may be achieved in accordancewith the independent claims of the present invention.

In one embodiment of the invention, a light element array is presentedand includes first, second, and third light emitting elements, and firstand second switches. The first light emitting element includes first andsecond terminals. The second light emitting element includes a firstterminal and a second terminal coupled to the second terminal of thefirst light emitting element. The third light element includes a firstterminal coupled to the first terminal of the first light emittingelement and a second terminal. The first switch includes a firstterminal coupled to each of the first terminals of the first and thirdlight emitting elements, and a second terminal coupled to the firstterminal of the second light emitting element. The second switchincludes a first terminal coupled to the second terminal of the thirdlight emitting element, and a second terminal coupled to each of thesecond terminals of the first and second light emitting elements.

In another embodiment of the invention, a light emitting device ispresented and includes a light element array as described above andherein, a power supply and a controller. The power supply includes acontrol input, and a power output coupled to supply current to the lightelement array. The controller includes a first output coupled to thecontrol input of the power supply, a second output coupled to firstswitch of the light element array, and a third output coupled to thesecond switch of the light element array, the first output operable toprovide a control signal to the power supply to set output levelconditions of the power supply, and the second output operable toprovide a control signal to control the state of the first switch, andthe third output operable to provide a control signal to control thestate of the second switch.

In a further embodiment of the invention, a method for operating a lightelement array is presented, the light element array as described aboveand herein, the method including the processes of selecting the firstlight emitting element, that operation involving controlling each ofsaid first and second switches to either an open state or a closedstate, and supplying current to the light element array. At least aportion of the supplied current is supplied to: (i) the first lightemitting element when each of the first and second switches are in anopen state, (ii) the second light emitting element when the first switchis in a closed state, and the second switch is in an open state, and(iii) the third light emitting element when the first switch is in anopen state, and the second switch is in a closed state.

It may be seen as a gist of an exemplary embodiment of the presentinvention implements light emitting elements having different operatingvoltage points (i.e., forward voltages for LEDs) in order to reduce thenumber of switches below the 1:1 ratio of switches to light emittingelements, for example, providing one switch for two light emittingelements, two switches for three light emitting elements, or twoswitches for four light emitting elements. In this manner, the componentcount for the light element array can be reduced, providing a faster,more power efficient and lower cost light emitting device.

The following describes exemplary features and refinements of the lightelement array, although these features and refinements will apply to thelight emitting device, and method of operating the light element arrayas well. In one embodiment, the first and second terminals of the firstlight element are coupled to first and second power supply rails.Further, the first, second, and third light emitting elements arecharacterized by respective first, second and third operating voltagesV_(OP1), V_(OP2), V_(OP3), the relative relationships defined such that,when the first switch is in a closed state and the second switch is inan open state, the second light emitting element is operable to receiveat least a portion of current supplied to the power supply rails. Saidrelationship being further defined, such that, when the first switch isin an open state and the second switch is in a closed state the thirdlight emitting element is adapted to receive at least a portion ofcurrent supplied to the power supply rails. In a particular embodiment,the relationship among the first, second, and third operating voltagesis defined as V_(OP1)>V_(OP2), V_(OP3). Such an arrangement in theoperating voltages allows selection between the various light emittingelements.

In a further embodiment, the light element array includes a fourth lightemitting element having a first terminal coupled to the second terminalof the first switch, and a second terminal coupled to the first terminalof the second switch, the fourth light emitting element characterized bya fourth operating voltage V_(OP4) at or above which the fourth lightemitting element is operable to emit light. In a specificimplementation, the fourth light emitting element is adapted to receiveat least a portion of current supplied to the light element array whenthe first and second switches are each in a closed state. Furtherspecifically, the first, second, third and fourth operating voltages(V_(OP1), V_(OP2), V_(OP3), V_(OP4)) are defined by the equation:V_(OP1)>V_(OP2), V_(OP3)>V_(OP4)

In a further embodiment, the first, second, third or fourth (whenimplemented) light emitting elements are selected from a groupconsisting of a light emitting diode, an organic light emitting diode,an AC light emitting diode, a laser diode or an incandescent light.Still further, a storage element, for example a capacitor, may beoptionally coupled to one or more of the first, second, third or fourth(when implemented) light emitting elements. The storage element may beused to increase the duration of illumination for one or more of thelight emitting elements, or to enable concurrent illumination of two ormore light emitting elements.

In a further embodiment, the fourth light emitting element (whenimplemented) includes at least one light emitting diode, and furthereach of the first, second and third light emitting elements include atleast one additional light emitting diode as included within the fourthlight emitting element, or is composed of a different semiconductormaterial than the at least one light emitting diode included within thefourth light emitting element. Such an arrangement can be used toprovide a lower operating voltage to the fourth light emitting element.

The following describes exemplary features and refinements of the methodfor operating the light element array, although these features andrefinements will apply to the light element array and light emittingdevice as well. In a further embodiment, currents supplied to the first,second, third and fourth light emitting elements are average currentsdetermined in accordance with the equation

${\overset{\_}{I}}_{i} = {I_{i} \cdot \frac{t_{i}}{T}}$

where I_(i) is the amplitude of the current available to the selectedi^(th) light emitting element, T is the time period of the currentsupplied to the i^(th) light emitting element, and t_(i) is anactivation period during which each of the first and second switches arein their respective states operable to supply current I_(i) to the ithlight emitting element.

The operations of the foregoing methods may be realized by a computerprogram, i.e. by software, or by using one or more specialelectronic/optimization circuits, i.e. in hardware, or inhybrid/firmware form, i.e. by software components and hardwarecomponents. The computer program may be implemented as computer readableinstruction code in any suitable programming language, such as, forexample, VHDL, assembler, JAVA, C++, and may be stored on acomputer-readable medium (removable disk, volatile or non-volatilememory, embedded memory/processor, etc.), the instruction code operableto program a computer or other such programmable device to carry out theintended functions. The computer program may be available from anetwork, such as the WorldWideWeb, from which it may be downloaded.

These and other aspects of the present invention will become apparentfrom and elucidated with reference to the embodiment describedhereinafter.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary embodiment of a light element array inaccordance with the present invention.

FIG. 2A illustrates a further exemplary embodiment of a light elementarray in accordance with the present invention.

FIG. 2B illustrates a further exemplary embodiment of a light elementarray in accordance with the present invention.

FIG. 2C illustrates a further exemplary embodiment of a light elementarray in accordance with the present invention.

FIG. 3 illustrates a method for operating the light element array shownin FIG. 1 and corresponding state table in accordance with the presentinvention.

FIG. 4A illustrates a method for operating the light element array shownin FIG. 2A and corresponding state table in accordance with the presentinvention.

FIG. 4B illustrates a method for operating the light element array shownin FIG. 2B and corresponding state table in accordance with the presentinvention.

FIG. 5 illustrates a light emitting device incorporating a light elementarray in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an exemplary embodiment of a light element array 100in accordance with the present invention. The array 100 includes a firstlight emitting element (LEE) 110 having a first terminal 110 a and asecond terminal 110 b, a second LEE 120 having a first terminal 120 aand a second terminal 120 b coupled to the second terminal 110 b of thefirst LEE 110, and a switch 130 having a first terminal 130 a coupled tothe first terminal 110 a of the first LEE 110, and a second terminal 130b coupled to the first terminal 120 a of the second LEE 120. As usedherein, the term “light emitting element” or “LEE” refers to any lightemitting element, circuit, device or component, such as, light emittingdiodes (LEDs), organic light emitting diodes (OLEDs), AC LEDs, laserdiodes, or any other lighting element, such as incandescent light andthe like.

The first terminal of the switch 130 and the first terminal of the firstLEE 110 are commonly coupled to a first power supply rail 172, and eachof the second terminals of the first and second LEEs are coupled to asecond supply rail 174. In a specific embodiment of the inventionfurther illustrated below, a current I_(supply) is supplied to the array100 via the supply rails.

Further optionally, the array 100 includes a storage element coupled toreceive energy from the power supply rails and to provide energy to oneor more of the LEEs 110 and 120. In the exemplary embodiment shown, acapacitor 160 is coupled across first LEE 110, the parallel-connectionof capacitor 160 and LEE 110 coupled in series with a decoupling element162. The decoupling element 162 may be implemented as a non-lightemitting diode, e.g. a Schottky diode with low forward voltage drop.Alternatively, the use of a light emitting element is possible. Thepurpose of the decoupling element 162 is to prevent discharge of thecapacitor 160 during selection of the second LEE 120. The capacitor 160is operable to provide power to the first LEE 110 during periods whenswitch 130 is closed. In another embodiment, the storage element 160 maybe an inductor coupled in series with LEE 110 and/or 120.

In a specific embodiment of the invention, the first and second LEEs 110and 120 are substantially operable at different bias conditions, e.g.,different operating voltages. Specifically, the first LEE 110 ischaracterized by a first voltage at or above which the first LEE 110 issubstantially operable to emit light. Similarly, the second LEE 120 ischaracterized by a second voltage at or above which the LEE 120 issubstantially operable to emit light. The relationship between the firstand second operating voltages V_(OP1) and V_(OP2) can be used to provideselectivity in operating LEE 110, and/or LEE 120. Specifically, therelationship between the first and second operating voltages V_(OP1) andV_(OP2) can be defined such that the second LEE 120 is adapted toreceive at least some of the energy supplied to the array 100 via thepower supply rails 172 and 174 when the first switch 130 is in a closedposition. In a particular embodiment, the first operating voltageV_(OP1) is higher than the second operating voltages V_(OP2).

In one exemplary embodiment, the LEEs 110 and 120 are circuits, each ofwhich includes at least one light emitting diode. In such an embodiment,each LEE 110, 120 may employ a plurality (i.e., 2, 3, 5, 10, or more) ofserial-coupled diodes, parallel-coupled diodes, or a combination ofserial and parallel coupled diodes. Furthermore, different materials maybe employed to fabricate the light emitting diodes, for example,Gallium-Nitride, Gallium-Phosphide, or other materials.

In one embodiment of the invention, the first operating voltage V_(OP1)of the first LEE 110 is greater than the second operating voltageV_(OP2) of the second LEE 120. This difference in operating voltages maybe accomplished through a variety of means. For example, in theembodiment in which LEEs 110 and 120 are LED circuits and LEE 110exhibits a higher operating voltage than LEE 120, the first LED circuit110 may include at least one additional series-coupled light emittingdiode in comparison with the light emitting diodes of the second LEDcircuit 120. In another example, different semiconductor materialsand/or processes may be used to fabricate the light emitting diodeswithin the first LED circuit 110 to have a higher forward voltagecompared to the forward voltage of the light emitting diodes within thesecond LED circuit 120. In another example, additional circuitcomponents (resistive divider, diodes etc.) may be used to provide thefirst LED circuit 110 with higher forward voltages compared to thesecond LED circuit 120. Those skilled in the art will appreciate that avariety of techniques may be used to impart a higher forward voltage tothe first LED circuit 110 in comparison to the second LED circuit 120.

FIG. 2A illustrates a further exemplary light element array 200 a inaccordance with the present invention, with previously described featureretaining their reference numerals. In this embodiment, the array 200 aincludes a third LEE 210, and a second switch 230.

The third LEE includes first and second terminals 210 a and 210 b, thefirst terminal 210 a coupled to a common node including the firstterminal 110 a of the first LEE 110, and the first terminal 130 a of thefirst switch 130. The second switch 230 includes a first terminal 230 acoupled to the second terminal of the third LEE 210, and a secondterminal 230 b coupled to the second terminals of the first and secondLEEs 110 and 120 and the second power supply rail 174.

The third LEE 210 is characterized by a third operating voltage(V_(OP3)) at or above which the third LEE 210 is substantially operableto emit light. The relationship among the first, second, and thirdoperating voltages V_(OP1), V_(OP2), V_(OP3) can be used to provideselectivity in operating the LEEs 110, 120, and 210. Specifically, therelationship among the first, second, and third operating voltagesV_(OP1), V_(OP2), V_(OP3) can be defined such that the second lightemitting element 120 is adapted to receive at least some of the energysupplied to the array 200 a via the power supply rails 172 and 174 whenthe first switch 130 is in an closed position and the second switch 230is in an open position, the third light emitting element 210 is adaptedto receive at least some of the energy supplied to the light elementarray 200 a via the power supply rails 172 and 174 when the first switch130 is in an open position and the second switch 230 is in a closedposition.

Further specifically, the relationship between the first, second, andthird operating voltages can be defined such that the second and thirdLEEs 120 and 210 are operable to receive substantially all of the energysupplied to the array during their corresponding selection by theaforementioned settings of the first and the second switch.

In a specific embodiment of the aforementioned operating voltagerelationships, the operating voltage V_(OP1) corresponding to the firstLEE 110 exhibits the highest operating voltage, resulting in thefollowing relationship:V_(OP1)>V_(OP2), V_(OP3)

As explained above, the operating voltages of the first and second LEEsV_(OP1) and V_(OP2) may be selected such that when the first switch isclosed and the second switch is open, the second LEE 120 receives atleast some current (the supply current in this case being shared withthe first LEE 110), and in a particular embodiment substantially all thesupply current I_(supply), when, for example, V_(OP2)<<V_(OP1), Furtherexemplary, the relationship of the first and third operating voltagesV_(OP1) and V_(OP3) can be arranged such that when the first switch isopen and the second switch is closed, the third LEE 210 receives atleast some of the supplied current (the supply current possibly beingshared with the first LEE 110), and in a particular embodiment, thethird LEE 210 receives substantially all of the supply current, when,e.g., V_(OP3)<<V_(OP1).

In the foregoing embodiments, three of four possible switching statesare used. Control of the array 200 a may be provided so as to precludeoperation in the fourth state in which both of the first and secondswitches 130 and 230 are in a closed state. Alternatively, the fourthswitching state may be optionally employed, whereby the current suppliedto each of the first, second and third LEEs 110, 120 and 210 isdetermined by the LEEs' corresponding operating voltages. For example,each of the three LEEs may receive substantially the same portion ofcurrent when the three operating voltages are substantially the same.Further exemplary of the aforementioned condition in whichV_(OP1)>>V_(OP2), V_(OP3), the first LEE 110 will receive the least (ifany) supply current, and the portions of supply current provided to thesecond and third LEEs 120 and 210 will depend upon the relationshipbetween their respective operating voltages. For example, ifV_(OP1)>>V_(OP2)≈V_(OP3), then each of the second and third LEEs 120 and210 will receive substantially the same portions of supply current, withthe first LEE 110 receiving little if any portion of the supply current.Further exemplary, if V_(OP1)>>V_(OP2)>V_(OP3), then the third LEE 210will receive the largest portion (and possibly all) of the supplycurrent, second LEE 120 receiving a lesser portion (and possibly none)of the supply current, and first LEE 110 receiving the smallest portion(and possible none) of the supply current. Accordingly, current can besupplied to any one or more LEEs in a particular amount by arranging theoperating voltages in a corresponding manner.

Similar to the array 100 of FIG. 1 the array 200 a may optionallyinclude one or more storage elements coupled to receive energy from thepower supply rails and to provide energy to one or more of the LEEs 110,120 and 210. In the exemplary embodiment shown, a capacitor 160 iscoupled across first LEE 110, the parallel-connection of capacitor 160and LEE 110 coupled in series with a decoupling element 162. Thedecoupling element 162 may be implemented as a non-light emitting diode,e.g. a Schottky diode with low forward voltage drop. Alternatively, theuse of a light emitting element is also possible. The purpose of thedecoupling element 162 is to prevent discharge of the capacitor 160during selection of LEE 120 or 210. The capacitor 160 is operable toprovide power to the first LEE 110 during periods when one or bothswitches 130 and 230 are closed. In another embodiment, the storageelement 160 may be an inductor coupled in series with LEE 110, 120,and/or 210

FIG. 2B illustrates a further exemplary embodiment of a light elementarray 200 b in accordance with the present invention, with previouslyidentified features retaining their reference indicia. The first, secondand third LEEs 110, 120 and 210 and switches 130 and 230 are coupled anddescribed above in FIG. 2A, and a fourth LEE 220 is coupled between thesecond terminal of the second switch 130, and the first terminal of thesecond switch 230, the fourth LEE 220 characterized by a operatingvoltage at or above which it will substantially operate to emit light.

The current supplied to the fourth LEE 220 in the fourth switching state(where the first and second switches are in the closed position) willdepend upon its relationship to the other three operating voltagesV_(OP1), V_(OP2) and V_(OP3). Continuing with the foregoing example inwhich the first operating voltage VOP1 is the highest, V_(OP1)>>V_(OP4),then the current delivered to the fourth LEE in the fourth switchingstate will depend upon the relationship of V_(OP4) to V_(OP2) andV_(OP3). For example, if V_(OP1)>>V_(OP2)≈V_(OP3)≈V_(OP4), then each ofthe second, third and fourth LEEs 120, 210 and 220 will receivesubstantially the same portions of supply current, with the first LEE110 receiving little if any portion of the supply current. The operatingvoltages can also be arranged, such that the fourth LEE 220 will receivemost, if not all of the supplied current, e.g., when V_(OP1)>>V_(OP2),V_(OP3)>>V_(OP4). In this manner, arrangement of the operating voltagescan be made so as to enable routing of the supply current in the desiredquantity to each of the first, second, third and fourth LEEs 110, 120,210 and 220.

The skilled person will appreciated that the LEEs may include thosefeatures described in connection with FIG. 2A above. For example, any ofthe one or more LEEs 110, 120, 210 and 220 illustrated in FIG. 2A may becoupled to a storage element, provided in one embodiment, as a parallelcoupled capacitor (and accompanying decoupling element 162), asdescribed and illustrated above.

The array 200 b with first, second, third and fourth LEEs 110, 120, 210and 220 may be modified to have a lower number of LEEs, e.g., three LEEsby omitting either the second, third or fourth LEEs 120, 210, or 220. Ina particular embodiment, the fourth LEE 220 is omitted, resulting in thearray 200 a illustrated in FIG. 2A.

FIG. 2C illustrates a further exemplary light element array 200 c inaccordance with the invention, in which previously described featuresretain their reference indicia. The first, second and third LEEs 110,120 and 210 and switches 130 and 230 are coupled and described above inFIG. 2A, and a short circuit is coupled between the second terminal ofthe second switch 130, and the first terminal of the second switch 230.This configuration may be used to provide a three element array (e.g.,in FIG. 2A) in which the fourth switch state is used as a deactivationstate for the array 200.

FIG. 3 illustrates an exemplary method for operating the light elementarray 100 shown in FIG. 1 and corresponding state table 350 inaccordance with the present invention, and in absence of energy storageelements, such as parallel-coupled capacitors 160. The method 300includes operation 312, where a supply current I_(supply) is set, and at314 a decision is made as to whether current is to be supplied to thesecond LEE 120 to enable it to emit light. At 316, the state of switch130 is set either to an open or a closed state, depending upon if LEE120 is to emit light; switch 130 controlled to an open state if LEE 120is to receive no supply current, or switch 130 controlled to a closedstate if LEE 120 is to be supplied at least some current to emit light.

The current I_(supply) set in operation 312 may provide a constantcurrent level or a modulated, time-varying level. If the array 100 is tooperate in state “0” in which the first LEE 110 emits light, the switch130 is controlled into the open position. The current I_(supply) will besupplied to the first LEE 110, thereby emitting light at its intendedintensity. As switch 130 is open, no current is supplied to the secondLEE 120, and accordingly no light produced thereby.

If the array 100 is to operate in state “1” in which the second LEE 120receives at least some current to emit light, the switch is controlledinto the closed position. Depending on the operating voltages of thefirst and the second LEE 110 and 120, the current may be conducted byone or both of the LEEs 110 and 120. In a particular embodiment of theinvention, the operating voltages of LEE 110 and LEE120 are defined suchthat LEE 120 conducts at least a portion of the supply currentI_(supply). Such an arrangement may be realized, e.g., by constructingthe second LEE 120 to have an operating voltage which is less than orequal to the operating voltage of the first LEE 110.

In another embodiment, operations 312-316 are performed such thatsubstantially all of the supplied current I_(supply) provided to thearray is supplied to the second LEE 120. Such an arrangement may berealized, for example using the aforementioned techniques ofconstructing the second LEE 120 to have a substantially lower operatingvoltage, and/or voltage limiting the supplied current I_(supply), thelimited voltage being sufficiently above the operating voltage of thesecond LEE 120, and sufficiently below the operating voltage of thefirst LEE 110.

The average current supplied to the LEEs may also be computed for aperiodic on-off sequence of the switch 130. In particular, currents I₁and 1₂ delivered to LEE110 and LEE 120 may be determined as:

${\overset{\_}{I}}_{i} = {I_{i} \cdot \frac{t_{i}}{T}}$

where: Ī_(i) is the average current to be supplied to the i^(th) LEE toachieve the LEE's

-   -   intended light level;    -   I_(i) is the amplitude of the current available to the i^(th)        LEE;    -   T is the time period of the current supplied to the i^(th) LEE    -   t_(i) is a portion of time within the period T during which        current I is supplied to    -   the i^(th) LEE.

In the particular embodiment of FIG. 1, t₁ for state “1” is the time theswitch 130 is kept in a closed state, resulting in an average currentsupplied to the second LEE 120 to operate it at the intended level. Thisapproach of providing an average value of current based upon switchingstate duration is further described for array embodiments of FIGS. 2Aand 2B below in FIGS. 4A and 4B.

The current amplitude I_(i) is the current level which is available forsupply to the selected LEE over activation period t₁. In one embodimentin which only the selected LEE conducts the current supplied to thearray, the current amplitude I_(i) will represent substantially the fullamplitude of I_(supply) provided to the array 100. In another embodimentin which one or more of the unselected LEEs conduct current, the supplycurrent I_(supply) will be divided, resulting in a current less thanI_(supply) being available for conduction by the selected LEE.Accordingly, the current amplitude I_(i) in the aforementioned equationrepresent the current amplitude which is available to a LEE upon itsselection by the array switches. Those skilled in art will appreciatethat for LEEs receiving energy when they are not selected, the averagecurrent is the sum of all current received during the selected and theunselected time periods.

Alternatively, current I_(supply) may itself be a varying current, e.g.a pulse width modulated (PWM) waveform to provide a particular quantityof current to the desired LEE 110 or 120, switch 130 being operable toselect which of the LEEs 110 or 120 current is supplied to. For example,when switch 130 is in an open state, the current may be supplied tofirst LEE 110 only. When switch is 130 is in a closed state, the currentis possibly supplied to both the first and second LEEs 110 and 120

If both LEE 110 and 120 are to be deactivated, the current supplying theLED array 100 is interrupted. This can be done by setting the current tozero in the operation 312. In this state, LEE 110 and LEE 120 will emitno light. In an alternative embodiment, the second LED circuits 120 maybe replaced with a short circuit, such that state “1” is selected toprovide no light output for the LED array 100. The skilled person willappreciate that operations 312, 314 and 316 may be performed in anyorder, and/or one or more of the operations may be performedconcurrently as well.

State table 350 illustrates selection states, switch states, andselected LEEs in accordance with one specific embodiment of theinvention in which the operating voltages are arranged such that LEE 120carries substantially all current delivered to the supply rails if theswitch 130 is in the closed state, and in which no storage element 160is used. As illustrated, in state “0”, switch 130 is in an open positionand the first LEE 110 is selected to receive current, said current beingthe supply current I_(supply) in one embodiment. In state “1,” switch130 is in a closed position, and in the particular embodiment shown LEE120 is selected to receive current, which in an exemplary embodimentrepresents substantially all of the supply current I_(supply). Asexplained above, the operating voltages V_(OP1) and V_(OP2) can beprovided such that LEE 110 and LEE 120 provide a particular on/offprofile, the illustrated embodiment of state 1 being representative ofthe aforementioned case in which V_(OP2) is substantially lower thanV_(OP1), and/or the supplied current I_(supply) is voltage limitedsufficiently above V_(OP2), and sufficiently below V_(OP1).Alternatively, the voltage operating points can be altered to permitconcurrent selection/light emission from both LEE 110 and LEE 120 instate 1, e.g., by providing the first and second LEEs 110 and 120 withsimilar operating voltages V_(OP1) and V_(OP2).

FIG. 4A illustrates a method 410 for operating the light element array200 a shown in FIG. 2A and corresponding state table 420 in accordancewith the present invention. Initially at 411, the level of currentI_(supply) to be applied to the light element array is set. At 412, adetermination is made as to which LEE 110, 120 and 210 is to beselected. If LEE 110 is to be selected to emit light, the processcontinues at 413, whereby both switches 130 and 230 are controlled to anopen state (or remains in an open state if presently there). Current isdelivered to the first LEE 110, which begins to emit light at itsintended light level.

If the array 200 a is to operate in state “01” in which the third LEE210 emits light, the process continues at 414, whereby the first switch130 is controlled to an open state, and the second switch 230 iscontrolled to a closed state. Current is delivered to the third LEE 210,which begins to emit light at its intended light level.

If the array 200 a is to operate in state “10” in which the second LEE120 emits light, the process continues at 415, whereby the first switch130 is controlled to a closed state, and the second switch 230 iscontrolled to an open state. Current is delivered to the second LEE 120,which begins to emit light at its intended light level.

The supply current I_(supply) may be provided as a constant current or amodulated current. In an example of the latter, the supply currentI_(supply) may be in the form of a pulse width modulated (PWM) waveformoperable to provide a particular quantity of current to the desired LEE110, 120, 210, whereby switches 130 and 230 are operable to select whichof the LEEs 110, 120, 210 current is to be supplied. The operatingvoltages of the LEEs 110, 120, and 210 are defined such that selectionbetween the LEEs can be accomplished by setting the aforementionedswitching states.

In a further exemplary embodiment, currents are provided to theirrespective LEEs as average currents described in FIG. 3 above. Inparticular, the operation of 413 of supplying current to the first lightemitting element 110 includes the operation of supplying an averagecurrent to the first light emitting element 110:

${\overset{\_}{I}}_{1} = {I_{1} \cdot \frac{t_{1}}{T}}$

where I₁ is the amplitude of the current available to the first LEE 110,T is the time period of the current supplied to the first LEE 110, andt₁ is the activation period within time period T during which each ofthe first and second switches 130 and 230 are in an open state to supplycurrent I₁ to the first LEE 110. The current amplitude I₁ may differ(but not necessarily) from the amplitude of the supply currentI_(supply), for example, when one or more of the unselected LEEs conductcurrent. Accordingly, the current amplitude I₁ in the aforementionedequation represent the current which is available to the first LEE 110upon its selection by the array switches.

Similarly, the operation of 414 of supplying a current to the second LEE120 may include the operation of supplying an average current to thesecond light emitting element 120:

$\overset{\_}{I_{2}} = {I_{2} \cdot \frac{t_{2}}{T}}$

where I₂ is the amplitude of the current available to the second LEE120, T is the time period of the current supplied to the second LEE 120,and t₂ is the activation period within time period T during which thefirst switch 130 is in a closed state and the second switch 230 is anopen state to supply current I₂ to the second LEE 120.

Similarly, the operation 415 of supplying current to the third LEE 210may include the operation of supplying an average current to the thirdlight emitting element 210:

$\overset{\_}{I_{3}} = {I_{3} \cdot \frac{t_{3}}{T}}$where I₃ is the amplitude of the current available to the third LEE 210,T is the time period of the current supplied to the third LEE 210, andt₃ is the activation period within time period T during which the firstswitch 130 is in an open state and the second switch 230 is a closedstate to supply current I₃ to the third LEE 210.

State table 420 illustrates selection states, switch states, andselected LEEs in accordance with one specific embodiment of theinvention in which LEE 110, LEE 120, or LEE 210 is supplied current, andin which no storage element 160 is used. As illustrated, in state “00,”switches 130 and 230 are in an open position and the first LEE 110 iscoupled to receive current. In state “01,” first switch 130 is in anopen position and second switch 230 is in a closed position, whereby thethird LEE 210 receives current for operation at it intended light level.In state “10,” first switch 130 is in a closed position and secondswitch 230 is in an open position, whereby second LEE 120 receivescurrent for operation at it intended light level.

As explained above, the operating voltages V_(OP1) V_(OP2) and V_(OP3)can be provided such that LEE 110, LEE 120, and LEE 210 provide aparticular on/off profile. In the foregoing embodiment in which state“01” result in substantially all of the supply current I_(supply) beingsupplied to the third LEE 210, V_(OP3) is substantially lower thanV_(OP1) and/or the supplied current I_(supply) is voltage limitedsufficiently above V_(OP3) and sufficiently below V_(OP1). Similarly, inthe embodiment in which state “10” result in substantially all of thesupply current I_(supply) being supplied to the second LEE 120, V_(OP2)is substantially lower than V_(OP1) and/or the supplied currentI_(supply) is voltage limited sufficiently above V_(OP2) andsufficiently below V_(OP1).

Alternatively, the voltage operating points V_(OP1), V_(OP2) and V_(OP3)can be altered to permit concurrent operation/light emission from LEE110 and LEE 120 or LEE 110 and LEE 210. For example, the first and thirdLEEs 110 and 210 may be both supplied current during operation in state“01,” e.g., in generally the same proportion if their correspondingoperating voltages V_(OP1) and V_(OP3) are similar, or in anotherproportion determined in correspondence with the relationship betweentheir operating voltages. Similarly, the first and second LEEs 110 and120 may be both supplied current during operation in state “10,” e.g.,in generally the same proportion if their corresponding operatingvoltages V_(OP1) and V_(OP2) are similar, or in another proportiondetermined in correspondence with he relationship between theiroperating voltages.

As explained above, the array's operation in a fourth possible statewhere the first and second switches 130 and 230 are in a closed statemay be precluded by an array controller (not shown). In still anotherembodiment, current supply I_(supply) to the array may be discontinuedif the array is operated in the fourth switch state.

Further exemplary, the fourth switching state may be employed, wherebythe current supplied to each of the first, second and third LEEs 110,120 and 210 is determined by the LEEs' corresponding operating voltages.For example, each of the three LEEs may receive substantially the sameportion of current when the three operating voltages are substantiallythe same. Exemplary of the aforementioned condition in whichV_(OP1)>>V_(OP2), V_(OP3), the first LEE 110 will receive the least (ifany) supply current, and the portions of supply current provided to thesecond and third LEEs 120 and 210 will depend upon the relationshipbetween their respective operating voltages. For example, ifV_(OP1)>>V_(OP2)≈V_(OP3), then each of the second and third LEEs 120 and210 will receive substantially the same portions of supply current, withthe first LEE 110 receiving little if any portion of the supply current.Further exemplary, if V_(OP1)>>V_(OP2)>V_(OP3), then the third LEE 210will receive the largest portion (and possibly all) of the supplycurrent, second LEE 120 receiving a lesser portion (and possibly none)of the supply current, and first LEE 110 receiving the smallest portion(and possible none) of the supply current. Accordingly, current can besupplied to any one or more LEEs in a particular amount by arranging theoperating voltages in a corresponding manner.

FIG. 4B illustrates a method 430 for operating the light element array200 b having a fourth LEE 220 as shown in FIG. 2B and correspondingstate table 450 in accordance with the present invention, withpreviously identified features retaining their reference indicia. Asillustrated in the method diagram 430, the previous operations ofsupplying current to the first, second and third LLEs 110, 120 and 210are as provided as described above. In this embodiment, the method ofoperation includes a further operation at 432, whereby if the fourth LEE220 is to be selected, the fourth state “11” is selected in which thefirst and second switches 130 and 230 are provided in a closed state.Current is delivered to the fourth LEE 220, which begins to emit lightat its intended level.

The operation 442 of supplying current to the fourth LEE 220 may includethe operation of supplying an average current to the fourth lightemitting element 220:

$\overset{\_}{I_{4}} = {I_{4} \cdot \frac{t_{4}}{T}}$

where I₄ is the amplitude of the current available to the fourth LEE220, T is the time period of the current supplied to the fourth LEE 220,and t₄ is the activation period within time period T during which eachof the first and second switches 130 and 230 are in a closed state tosupply current I₄ to the fourth LEE 220.

As earlier described, the current supplied to the fourth LEE 220 in thefourth switching state of the array illustrated in FIG. 2B will dependupon its relationship to the other three operating voltages V_(OP1),V_(OP2) and V_(OP3). Referring to the foregoing example in which thefirst operating voltage V_(OP1) is the highest, V_(OP1)>>V_(OP4), thenthe current delivered to the fourth LEE in the fourth switching statewill depend upon the relationship of V_(OP4) to V_(OP2) and V_(OP3). Forexample, if V_(OP1)>>V_(OP2)≈V_(OP3)≈V_(OP4), then each of the second,third and fourth LEEs 120, 210 and 220 will receive substantially thesame portions of supply current, with the first LEE 110 receiving littleif any portion of the supply current. The operating voltages can also bearranged, such that the fourth LEE 220 will receive most, if not all ofthe supplied current, e.g., when V_(OP1)>>V_(OP2), V_(OP3)>>V_(OP4). Inthis manner, arrangement of the operating voltages can be made so as toenable routing of substantially all of supply current I_(supply) to thefourth LEE 220 when the array operates in the fourth switching state.

FIG. 5 illustrates a light emitting device 500 incorporating the lightelement array 100 or 200 a, 200 b, 200 c shown in FIGS. 1, 2A, 2B or 2Cin accordance with the present invention, with previously-describedfeatures retaining their reference numerals. FIG. 5 is shownimplementing the array 200 b of FIG. 2B, although the skilled personwill appreciate that the array 100 shown in FIG. 1, or the arrays of 200a or 200 c in FIGS. 2A and 2C, respectively may be alternativelyemployed in the light emitting device 500.

In addition to the array 200 b, the light emitting device 500 furtherincludes a power supply 510 The power supply 510 includes a power input510 a, a control input 510 b, and a current output 510 c coupled(directly, or via switches) to the array 200 b. The power input 510 a isoperable to receive power (regulated or un-regulated) which is to besupplied to the array 200 b. Control input 510 b is operable to set theoutput level conditions, as will be described below. Current output 510c is operable to supply the power supply current I_(supply) to the array200. In a specific embodiment, the power supply 510 is operable as aconstant-current source, whereby the supply current I_(supply) deliveredis substantially independent of the loading conditions.

The LED device 500 further includes a controller 520 operable to providecontrol signals to the power supply 510 and the first and the secondswitch 130 and 230 The controller 520 includes a first output 520 acoupled to the control input 510 a of the power supply 510, and a secondoutput 520 b coupled to switch 130. The first output 520 a is operableto provide a control signal 522 to the power supply to set output levelconditions of the power supply. The second output 520 b is operable toprovide a control signal 524 to control the state of the first switch130. In the illustrated embodiment in which the array 200 b of FIG. 2Bis implemented, the controller 520 further includes a third output 520 ccoupled to the second switch 230, the third output 520 c operable toprovide a control signal 526 to control the state of the second switch230. The controller includes an input port at 520 d operable to receiveinstructions from a microprocessor or another system. Alternatively, thecontroller 520 itself may be loaded with programming operable to performthe operations as described herein.

As those skilled in the art will appreciate, the current/power supply510 can be controlled to provide a variety of different output signalsto select and/or deactivate the LEEs 110, 120, 210 and 220. For example,in order to obtain constant illumination of an LEE circuit, controller520 may be made operable to provide a control signal 522 instructing thepower supply 510 to output a constant current at a level correspondingto the LEE circuit to be illuminated, the controller 520 furtherproviding controls signals 524 and 526 to set switches 130 and 230 tocouple the constant current to the desired LEE in accordance with table350 or 450 as described above. In a similar manner, the power supply 510may be controlled to vary the level of the output current I_(supply) toeffectuate a change in the LEE's intensity or luminance.

In another embodiment, the current supplied to each of the LEEs 110,120, 210 and 220 is in the form of an average current Ī_(i) described inFIGS. 3, 4A and 4B above. In one example of such an embodiment, the timeperiod T is 10 ms, and activation times t₁, t₂, t₃, and t₄ are 5 ms, 2ms, 2 ms, and 1 ms, respectively. Such an arrangement results in theaverage current Ī_(1,2,3,4) of the first, second, third, and fourth LEEsbeing 50%, 20%, 20% and 10% of the supply current level I_(supply),assuming that there is no substantial different between the currentI_(i) which is available to the selected LEE and the supply currentI_(supply) (i.e., no substantial conduction by non-selected LEEsoccurs). Those skilled in the art will appreciate that different currentratios can be obtained, depending upon the desire amount of currentwhich is to be supplied to a particular LEE. For example, the timeperiod T may be chosen so as to further avoid the human perception offlickering, in which case a shorter time period (e.g., 2.5 ms) may beselected. Furthermore, the activation time t_(i) may also becorrespondingly varied to maintain a percentage of the current levelI_(supply).

It is further noted that the supply current level I_(supply) may beprovided at a constant level, or alternatively, at a varying level.Combining the different states to select the LEE with the possibility tovary the supply current level allows two substantially independentdegrees of freedom in controlling the LEEs.

In a further exemplary embodiment, the array 200 b employs shuntcapacitor(s) coupled across one or more of the LEE circuits 110, 120,210 and 220 (LEE circuit 120 illustrating a shunt capacitor 160 in FIG.5, although several or all LEEs may employ shunt capacitors), the array200 b operating to provide an average current Ī_(i), to theircorresponding LEEs 110, 120, 210 and 220, such as in the example abovein which average currents Ī_(1,2,3,4) are provided at 5 ms, 2 ms, 2 ms,and 1 ms, respectively over a time period T of 10 ms. Providingparallel-coupled capacitors to one or more of the LEEs may be used toprovide continuous illumination of a particular LEE circuit for a periodof time, or to permit concurrent illumination of two or more LEEs, thelatter condition arising, for example, when a previously-inactive LEE isswitched to receive its corresponding Ī_(i), and a second LEE isdecoupled from the power supply, the second LEE's shunt capacitorproviding current to drive its LEE for continued operation.

The size of the capacitors 160 (each of which may be the same ordifferent) is based upon several factors, including the time period T ofthe current Ī_(i), the acceptable magnitude of rippled within thecurrent Ī_(i) delivered to the coupled LEE, and the duration of “offstate operation,” “off state operation” referring to the condition inwhich the stored charge of a coupled capacitor 160 operates thecorresponding LEE after switches 130 and 230 switch the LEE out of thepower supply. As will be appreciated, smaller capacitors can be employedwhen current Ī_(i) includes a shorter time period, and/or when the offstate activation time is shorter, and/or when a larger magnitude ofripple in Ī_(i) is desired or acceptable. A larger capacitance may beemployed in instances in which a longer time period T is provided bycurrent Ī_(i), and/or when a longer off state activation period issought, and/or when a smaller ripple magnitude in Ī_(i) is desired orrequired.

Another factor possibly impacting the size selection for capacitor(s)160 is the acceptable delay in selecting or deactivating the LEDcircuits employing shunt capacitor 160. In particular, the size of thecapacitor 160 may inhibit how fast a previously-inactive LED circuit canreach its operating voltage condition V_(OP), or how fast apreviously-active LEE can be deactivated. In such circumstances, therise and fall time transitions between inactive and active states of thesupplied average current Ī_(i) can be degraded beyond an acceptablelimit, resulting in erroneous emission of light in some circumstances(delayed deactivation of an LED circuit), and/or the omission of lightin other circumstance (delayed selection of an LEE).

One exemplary approach for minimizing the delayed selection/deactivationeffects which the shunt capacitor(s) 160 have on their coupled LEEs isto provide an intermittent compensation effect to accelerate the riseand fall time transitions. For example, the rise time transition of apreviously-inactive LEE to an active state can be accelerated byproviding, for a short period of time, a higher current level to theLEE, thereby charging its shunt capacitor 160 faster and achieving theforward voltage sooner than if the desired current level I_(i) isapplied constantly over time t during which the particular LEE circuitis active. Due to the certain voltage-current characteristics of certainLEEs, for example an LED which might be used as a light emittingelement, with lower forward voltages the forward current might drop.Discharging the capacitor by delivering the energy to the LED may resultin a long time period in which only very little but noticeable light isproduce by the LED. Connecting an additional load with appropriatecharacteristics (e.g. resistor or a serial connection of a resistor anda zener-diode) can be used to speed up the final off-state of the diode.In addition, the controller 520 can be programmed in a way to compensatethe missing or additional light output from a LED with a shunt capacitorand can compensate this with respect to the time averaged light output.

In summary it may be seen as one aspect of the present invention thatLEE circuits having different operating voltage points (i.e., turn onvoltages) are employed in order to reduce the number of switches belowthe 1:1 ratio of switches to light emitting elements, for example,providing one switch for two light emitting elements, two switches forthree light emitting elements, or two switches for four light emittingelements. In this manner, the component count for the light elementarray can be reduced, providing a faster, more power efficient and lowercost light emitting device.

As readily appreciated by those skilled in the art, the describedprocesses may be implemented in hardware, software, firmware or acombination of these implementations as appropriate. In addition, someor all of the described processes may be implemented as computerreadable instruction code resident on a computer readable medium(removable disk, volatile or non-volatile memory, embedded processors,etc.), the instruction code operable to program a computer of other suchprogrammable device to carry out the intended functions.

It should be noted that the term “comprising” does not exclude otherfeatures, and the definite article “a” or “an” does not exclude aplurality, except when indicated. It is to be further noted thatelements described in association with different embodiments may becombined. It is also noted that reference signs in the claims shall notbe construed as limiting the scope of the claims. The term “coupling” isused to indicate either a direct connection between two features, or anindirect connection, via an intervening structure, between two features.Operations illustrated in flow charts are not limited to the particularsequence shown, and later numbered operations may be performed currentlywith, or in advance of earlier number operations in accordance with theinvention.

The foregoing description has been presented for purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the disclosedteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined solely by the claims appended hereto.

1. A light element array, comprising: a first light emitting elementhaving a first terminal and a second terminal; a second light emittingelement having a first terminal, and a second terminal coupled to thesecond terminal of the first light emitting element; a third lightemitting element having a first terminal coupled to the first terminalof the first light emitting element and a second terminal; a firstswitch having a first terminal coupled to each of the first terminals ofthe first and third light emitting elements, and a second terminalcoupled to the first terminal of the second light emitting element; anda second switch having a first terminal coupled to the second terminalof the third light emitting element, and a second terminal coupled toeach of the second terminals of the first and second light emittingelements, wherein the first terminal of the first light emitting elementis coupled to a first power supply rail and the second terminal of thefirst light emitting element is coupled to a second power supply rail,and the first, second, and third light emitting elements arecharacterized by respective first, second, and third operating voltages(VOP1, VOP2, VOP3), the relative relationships of which is defined bythe equation:V_(OP1)>V_(OP2),V_(OP3) the second light emitting element being adaptedto receive at least a portion of current supplied to the first or secondpower supply rails when the first switch is in a closed state and thesecond switch is in an open state; and the third light emitting elementbeing adapted to receive at least a portion of current supplied to thelight element array when the first switch is in an open state and thesecond switch is in a closed state.
 2. The light element array of claim1, wherein each of the first, second, and third light emitting elementsis selected from a group consisting of a light emitting diode, anorganic light emitting diode, an AC light emitting diode, a laser diodeand an incandescent light.
 3. The light element array of claim 1,further comprising a storage element coupled to one or more of thefirst, second, or third light emitting elements.
 4. The light elementarray of claim 3, wherein the storage element comprises a capacitancecoupled in parallel with one or more of the first, second, or thirdlight emitting elements.
 5. The light element array of claim 1, furthercomprising a fourth light emitting element having a first terminalcoupled to the second terminal of the first switch, and a secondterminal coupled to the first terminal of the second switch, the fourthlight emitting element characterized by a fourth operating voltage(VOP4) at or above which the fourth light emitting element is operableto emit light.
 6. The light element array of claim 5, wherein the firstterminal of the first light emitting element is coupled to a first powersupply rail and the second terminal of the first light emitting elementis coupled to a second power supply rail, and wherein the fourth lightemitting element is adapted to receive at least a portion of currentsupplied to the light element array (200 b) when the first and secondswitches are each in a closed state.
 7. The light element array of claim5, wherein the first, second, third and fourth light emitting elements(110, 120, 210, 220) are characterized by respective first, second,third and fourth operating voltages (VOP1, VOP2, VOP3, VOP4), therelative relationships of which is defined by the equation:V_(OP1)>V_(OP2),V_(OP3)>V_(OP4.)
 8. The light element array of claim 5,further comprising a storage element coupled to the fourth lightemitting element.
 9. The light element array of claim 8, wherein thestorage element comprises a capacitance coupled in parallel with thefourth light emitting element.
 10. The light element array of claim 1,further comprising a short circuit coupled between the second terminalof the first switch, and the first terminal of the second switch.
 11. Alight emitting device, comprising: the light element array as claimed inclaim 1; a power supply (510) having a control input (510 b), and apower output (510 c) coupled to supply current to the light elementarray; and a controller (520) having a first output (520 a) coupled tothe control input (510 b) of the power supply (510), a second output(520 b) coupled to first switch of the light element array, and a thirdoutput (520 c) coupled to the second switch of the light element array(200), the first output (520 a) operable to provide a control signal(522) to the power supply (510) to set output level conditions of thepower supply (510), and the second output (520 b) operable to provide acontrol signal (524) to control the state of the first switch, and thethird output (520 c) operable to provide a control signal 526 to controlthe state of the second switch.
 12. A method for operating a lightelement array, the light element array including a first light emittingelement having a first terminal coupled to a first power supply rail anda second terminal coupled to a second power supply rail, a second lightemitting element having a first terminal, and a second terminal coupledto the second terminal of the first light emitting element, a thirdlight emitting element having a first terminal coupled to the firstterminal of the first light emitting element and a second terminal, afirst switch having a first terminal coupled to each of the firstterminals of the first and third light emitting elements, and a secondterminal coupled to the first terminal of the second light emittingelement, and a second switch having a first terminal coupled to thesecond terminal of the third light emitting element, and a secondterminal coupled to each of the second terminals of the first and secondlight emitting elements, the method comprising: controlling each of saidfirst and second switches to either an open state or a closed state, andsupplying current to the first or second power supply rail of the lightelement array, wherein at least a portion of the supplied current issupplied to the first light emitting element when each of the first andsecond switches is in an open state, wherein at least a portion of thesupplied current is supplied to the second light emitting element whenthe first switch is in a closed state, and the second switch is in anopen state; and wherein at least a portion of the supplied current issupplied to the third light emitting element when the first switch is inan open state, and the second switch is in a closed state.
 13. Themethod of claim 12, wherein the current supplied to the first lightemitting element comprises an average current:$\overset{\_}{I_{1}} = {I_{1} \cdot \frac{t_{1}}{T}}$ where I1 is theamplitude of the current supplied to the first light emitting element, Tis the time period of the current supplied to the first light emittingelement, and t1 is the activation period during which each of the firstand second switches are in an open state to supply current I1 to thefirst light emitting element.
 14. The method of claim 12, wherein thecurrent supplied to the second light emitting element comprises anaverage current: $\overset{\_}{I_{2}} = {I_{2} \cdot \frac{t_{2}}{T}}$where I2 is the amplitude of the current supplied to the second lightemitting element, T is the time period of the current supplied to thesecond light emitting element and t2 is the activation period duringwhich the first switch is in a closed state and the second switch is anopen state to supply current I2 to the second light emitting element.15. The method of claim 12, wherein the current supplied to the thirdlight emitting element comprises an average current:$\overset{\_}{I_{3}} = {I_{3} \cdot \frac{t_{3}}{T}}$ where I3 isamplitude of the current supplied to the third light emitting element Tis the time period of the current supplied to the third light emittingelement, and t3 is the activation period during which the first switchis in an open state and the second switch is a closed state to supplycurrent I3 to the third light emitting element.
 16. The method of anyone of claim 12, wherein the light element array further includes afourth light emitting element having a first terminal coupled to thesecond terminal of the first switch, and a second terminal coupled tothe first terminal of the second switch and wherein controllingcomprises controlling each of the first and second switches to a closedstate, wherein at least a portion of the supply current is supplied tothe fourth light emitting element.
 17. The method of claim 16, whereinthe current supplied to the fourth light emitting element comprises anaverage current: $\overset{\_}{I_{4}} = {I_{4} \cdot \frac{t_{4}}{T}}$where I4 is the amplitude of the current supplied to the fourth lightemitting element, T is the time period of the current supplied to thefourth light emitting element, and t4 is the activation period duringwhich the first and second switches are in a closed state to supplycurrent I4 to the fourth light emitting element.